Field of the Invention
The present invention relates to a radiation tube, and a radiation generating apparatus and a radiation imaging system including the radiation tube, which can be used for medical equipment, a nondestructive inspection apparatus, and the like, for example.
Description of the Related Art
A radiation tube is used for accelerating, in a vacuum with a high voltage, electrons emitted from an electron source, and irradiating a target formed of a metal with the electrons to generate radiation such as X-rays. The high voltage applied in the radiation tube is required to be, for example, about 100 kV. When particulate foreign matter exists in vacuum space with a high electric field due to such a high voltage, discharge due to the foreign matter (foreign matter discharge) sometimes occurs. Foreign matter discharge is a phenomenon in which charged foreign matter in the radiation tube exchanges charge at a cathode and an anode and, while reciprocating with force applied thereto from the high electric field, stochastically discharges in collision with the cathode and the anode.
One origin of the foreign matter is foreign matter that enters the radiation tube in a process of assembling the radiation tube. Generation of such foreign matter can be reduced by washing members and an assembly jig and cleaning an assembly process environment. Another origin of the foreign matter is foreign matter separated from a member in the radiation tube, which is caused when the radiation tube is driven. For example, when the anode is irradiated with an electron beam during the drive of the radiation tube, a member inside the radiation tube is damaged by generated heat to be separated. Such separation can be inhibited by reconsidering drive conditions and structure design. As described above, measures for inhibiting entrance and generation of foreign matter can be taken. On the other hand, it is undeniable that, depending on instability of the process and fluctuations in drive conditions, foreign matter enters or is generated accidentally.
Japanese Patent Application Laid-Open No. 2013-101879 discloses a structure in which, by covering, with a dielectric, a joint portion between a tubular member forming the radiation tube and the cathode or the anode, electric field concentration that occurs at the joint portion is caused to be less liable to occur to inhibit discharge.
However, in the structure described above, foreign matter that enters or is generated in the radiation tube itself is not eliminated, and thus, there is still a possibility that foreign matter discharge occurs.